Editing Mineral (section)

===Modern classification===
{{See also|Abundance of elements in Earth's crust}}

Minerals are classified by variety, species, series and group, in order of increasing generality. The basic level of definition is that of mineral species, each of which is distinguished from the others by unique chemical and physical properties. For example, quartz is defined by its [[Chemical formula|formula]], SiO<sub>2</sub>, and a specific [[Crystal structure|crystalline structure]] that distinguishes it from other minerals with the same chemical formula (termed [[Polymorphism (materials science)|polymorphs]]). When there exists a range of composition between two minerals species, a mineral series is defined. For example, the [[biotite]] series is represented by variable amounts of the [[endmembers]] [[phlogopite]], [[siderophyllite]], [[annite]], and [[eastonite]]. In contrast, a mineral group is a grouping of mineral species with some common chemical properties that share a crystal structure. The [[pyroxene]] group has a common  formula of XY(Si,Al)<sub>2</sub>O<sub>6</sub>, where X and Y are both cations, with X typically [[ionic radius|bigger]] than Y; the pyroxenes are single-chain silicates that crystallize in either the [[orthorhombic]] or [[monoclinic]] crystal systems. Finally, a mineral variety is a specific type of mineral species that differs by some physical characteristic, such as colour or crystal habit. An example is [[amethyst]], which is a purple variety of quartz.<ref name="DG20-22">{{harvnb|Dyar|Gunter|2008}}, pp. 20–22</ref>

Two common classifications, Dana and Strunz, are used for minerals; both rely on composition, specifically with regards to important chemical groups, and structure. [[James Dwight Dana]], a leading geologist of his time, first published his ''System of Mineralogy'' in 1837; {{As of|1997|lc=y}}, it is in its eighth edition. The [[Dana classification]] assigns a four-part number to a mineral species. Its class number is based on important compositional groups; the type gives the ratio of cations to anions in the mineral, and the last two numbers group minerals by structural similarity within a given type or class. The less commonly used [[Strunz classification]], named for German mineralogist [[Karl Hugo Strunz]], is based on the Dana system, but combines both chemical and structural criteria, the latter with regards to distribution of chemical bonds.<ref>{{harvnb|Dyar|Gunter|2008}}, pp 558–59</ref>

As the composition of the Earth's crust is dominated by silicon and oxygen, silicates are by far the most important class of minerals in terms of rock formation and diversity. However, non-silicate minerals are of great economic importance, especially as ores.<ref>{{harvnb|Dyar|Gunter|2008}}, p. 641</ref><ref name="{{harvnb|Dyar|Gunter|2008}}, p. 681">{{harvnb|Dyar|Gunter|2008}}, p. 681</ref> Non-silicate minerals are subdivided into several other classes by their dominant chemistry, which includes native elements, sulfides, halides, oxides and hydroxides, carbonates and nitrates, borates, sulfates, phosphates, and organic compounds. Most non-silicate mineral species are rare (constituting in total 8% of the Earth's crust), although some are relatively common, such as calcite, [[pyrite]], [[magnetite]], and [[hematite]]. There are two major structural styles observed in non-silicates: close-packing and silicate-like linked tetrahedra. [[Close-packing of equal spheres|Close-packed structures]] are a way to densely pack atoms while minimizing interstitial space. Hexagonal close-packing involves stacking layers where every other layer is the same ("ababab"), whereas cubic close-packing involves stacking groups of three layers ("abcabcabc"). Analogues to linked silica tetrahedra include {{Chem|S|O|4|4-}} ([[sulfate]]), {{Chem|P|O|4|4-}} ([[phosphate]]), {{Chem|As|O|4|4-}} ([[arsenate]]), and {{Chem|V|O|4|4-}} ([[vanadate]]) structures. The non-silicates have great economic importance, as they concentrate elements more than the silicate minerals do.<ref>{{harvnb|Dyar|Gunter|2008}}, pp. 641–43</ref>

The largest grouping of minerals by far are the [[Silicate minerals|silicates]]; most rocks are composed of greater than 95% silicate minerals, and over 90% of the Earth's crust is composed of these minerals.<ref name="{{harvnb|Dyar|Gunter|2008}}, p. 104">{{harvnb|Dyar|Gunter|2008}}, p. 104</ref> The two main constituents of silicates are silicon and oxygen, which are the two most abundant elements in the Earth's crust. Other common elements in silicate minerals correspond to other common elements in the Earth's crust, such as aluminium, magnesium, iron, calcium, sodium, and potassium.<ref>{{harvnb|Dyar|Gunter|2008}}, p. 5</ref> Some important rock-forming silicates include the [[feldspar]]s, quartz, [[olivine]]s, [[pyroxene]]s, [[amphibole]]s, [[garnet]]s, and [[mica]]s.